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When asteroids attack

An asteroid as big as a city block smashed into what is now northern Iowa about 470 million years ago, says a Smithsonian geologist, supporting a theory that a giant space rock broke up and bombarded Earth just as early life began flourishing in the oceans.

Meteorites sit on a display table at the Deep Space Industries announcement of plans for the world's first fleet of commercial asteroid-prospecting spacecraft at the Museum of Flying in Santa Monica, California, Jan.22.

By:Brian VastagThe Washington Post, Published on Sat Feb 23 2013

WASHINGTON—An asteroid as big as a city block smashed into what is now northern Iowa about 470 million years ago, says a Smithsonian geologist, supporting a theory that a giant space rock broke up and bombarded Earth just as early life began flourishing in the oceans.

The impact dug a crater nearly 6.5 kilometres wide that now lies beneath the town of Decorah, said Bevan French, one of the world’s foremost crater hunters and an adjunct scientist at the U.S. National Museum of Natural History.

The Decorah object smashed into bedrock with such force that it shattered tiny grains of minerals. French found this “shock quartz” in gravel from beneath the town, he told two dozen colleagues during a recent seminar at the museum.

Finding impact craters is rare, as erosion and the shifting of tectonic plates tend to erase them. The Decorah crater, if accepted by other scientists, would be just the 184th known, according to an international database.

But spying the evidence of the Earth’s most dramatic explosions requires only humble equipment — a simple black microscope.

While peering at a thin slice of rock from Decorah on a glass slide, three white circles — quartz crystals no bigger than mustard seeds — popped into view. Dozens of parallel lines striped each circle: evidence of a rock-crushing pulse.

“They’re shattered,” French said of the crystals. Geologists consider shock quartz near-definitive evidence of an extraterrestrial impact.

The Decorah crater lay undiscovered until now because almost none of it peeks above ground. Instead, it is filled by an unusual shale that formed after an ancient seaway sluiced into the crater, depositing sediment and an array of bizarre sea creatures that hardened into fossils, French said.

Unusual rock

Jean Young, an amateur geologist, noticed the shale about 12 years ago, when inspecting gravel pulled up by well-drilling machines. It looked like no other rock she had seen in the region.

Young sent samples to Robert McKay, a geologist at the Iowa Department of Natural Resources. McKay then pulled from his files “churn gravel” from dozens of other wells drilled in a four-township area centred on Decorah.

He found the same shale in some of the other samples. When plotted on a map, the shale-rich borings described a “nice circular basin,” almost completely encompassing Decorah, he said.

McKay and colleagues dubbed the black rock Winnesheik Shale and published a scientific paper describing it in 2006. But only after French recently identified the shock quartz — which was pulled from beneath the shale — did a giant impact seem more certain.

“They found what would be expected from an impact,” said Michael Velbel, a Michigan State University geologist, on sabbatical at the Smithsonian. “It’s clear the shale fills a crater.”

Fossils in the shale — including eel-like conodonts, worms called vermiforms, and shrimp-like creatures called eurypterids — date the crater to about 470 million years ago, McKay said. This geologic period, known as the Middle Ordovician, was marked by an explosion of early life in the oceans.

Massive collision

This period was also marred by an apparent uptick in the number of asteroid impacts on Earth. About a dozen of the planet’s known impact craters hail from that time.

In 2004, astronomers proposed a shocking explanation: A massive collision in the asteroid belt beyond Mars about 469 million years ago bombarded Earth with asteroid fragments.

The Decorah crater may have been formed by one such fragment, French said. Even more intriguingly, this new-found crater lies on a line between two other impacts of roughly the same age: the Rock Elm crater in Wisconsin and the Ames crater in Oklahoma.

A single giant asteroid may have flown in from the south, shattered and left a pockmarked trail of smouldering craters strung along about 800 kilometres.

French labelled the possibility “stimulating speculation.”

Brian Vastag is a reporter for The Washington Post.

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